The present invention relates to a distributed power supply system with a single operation detection function used between an electric power system and a power generating system such as a solar cell power generating system, a fuel cell power generating system, or wind power generating system. The single operation detection function is used to detect when the power generating system of the distributed power supply system is in single operation, without the electric power system.
Examples of a system configuration of a related system interconnecting inverter and a related single operation detecting unit are shown in FIG. 3 (see Japanese Patent No. 3353549). In the systems shown in FIG. 3, periodic minute fluctuations are given to a set value of a reactive power setting unit provided in a system interconnecting inverter in system interconnection to always periodically fluctuate reactive power at a system interconnecting point to thereby monitor the frequency of the signal at the system interconnecting point or the frequency of the signal of a system interconnecting inverter.
When a minute increase in the monitored frequency is detected, the generation of a minute signal in phase advance to the phase of the signal of the system interconnecting inverter further increases the monitored frequency when the system is in a single operation state. While, when a minute decrease in the monitored frequency is detected, the generation of a minute signal in phase delay to the phase of the signal of the system interconnecting inverter further decreases the monitored frequency when the system is in a single operation state. By giving attention to the phenomenon, a single operation phenomenon is to be surely and promptly detected.
In FIG. 3, a fuel cell power generation system 1 is formed of a fuel cell main unit 2, a system interconnecting inverter 3 and a distribution transformer 4. Reference numerals 5 and 6 denote a load and an electric power system, respectively. Moreover, reference numeral 7 denotes a circuit breaker for electric power distribution to the load 6, reference numeral 8 denotes a distributed power supply side circuit breaker for the fuel cell power generation system 1 as a distributed power supply, and reference numeral 9 denotes a power receiving point circuit breaker for receiving power from the distribution feeder of the electric power system 6.
Furthermore, a single operation detecting system 10 is formed of a frequency detecting circuit 11, including components such as a frequency to voltage converter, and a single operation detecting unit 20. The single operation detecting unit 20 is formed of a fluctuating signal generating circuit 21, a frequency increase monitoring circuit 22, a frequency decrease monitoring circuit 23, an OR circuit 24, a Schmitt circuit 25, a counter circuit 26, a negative polarity peak hold circuit 27, a positive polarity peak hold circuit 28, a switching circuit 29, a frequency upper limit monitoring circuit 30, a frequency lower limit monitoring circuit 31 and a single operation signal generating circuit 32.
Here, the fluctuating signal generating circuit 21 generates a periodic sinusoidal-wave-like minute signal. Each of the frequency increase monitoring circuit 22 and the frequency decrease monitoring circuit 23 includes a comparator and a setting unit, both for monitoring minute fluctuation of the value of the output of the frequency detecting circuit 11. The OR circuit 24 detects the operation of either of the frequency increase monitoring circuit 22 or the frequency decrease monitoring circuit 23. The Schmitt circuit 25 converts the output of the fluctuating signal generating circuit 21 to a pulsed wave signal. The counter circuit 26 carries out counting of the pulsed wave signal. The negative polarity peak hold circuit 27 is operated as a phase advanced signal generating circuit holding the negative polarity peak value of the output signal of the fluctuating signal generating circuit 21. The positive polarity peak hold circuit 28 is operated as a phase delayed signal generating circuit holding the positive polarity peak value of the output signal of the fluctuating signal generating circuit 21. The switching circuit 29 includes components such as an AND/OR gate circuit and three state buffers and, when any one of the frequency increase monitoring circuit 22, the frequency decrease monitoring circuit 23 and the counter circuit 26 outputs a signal, carries out switching so that one of the fluctuating signal generating circuit 21, the negative polarity peak hold circuit 27 and the positive polarity peak hold circuit 28, corresponding to the circuit outputting the above signal, is selected to output a signal by which minute fluctuation is given to a set value of an unillustrated reactive power setting unit in the system interconnecting inverter 3. Each of the frequency upper limit monitoring circuit 30 and the frequency lower limit monitoring circuit 31 includes a comparator and a setting unit for always giving the reactive power at a power receiving point A minute fluctuation and monitoring the deviation of the output frequency of the frequency detecting circuit 11 from the specified reference frequency. The single operation signal generating circuit 32 includes an OR gate generating a single operation signal when either the frequency upper limit monitoring circuit 30 or the frequency lower limit monitoring circuit 31 is operated.
FIG. 4 is a waveform diagram showing the operation of the system shown in FIG. 3.
In FIG. 4, when the electric power system 6 is disconnected by the power receiving point circuit breaker 9 due to an accident on the electric power system side at the time t0 shown in (B) in FIG. 4 with an amount of generated power supplied from the system interconnecting inverter 3 in the fuel cell power generation system 1 being balanced with an amount of the power consumed in the load 5, a single operation phenomenon is caused between the fuel cell power generation system 1 and the load 5.
In such a state, with a setting provided so that the fluctuating signal generating circuit 21 and the switching circuit 29 give a reactive power fluctuation to the system interconnecting inverter 3 and, as shown in (A) in FIG. 4, the fuel cell power generation system 1 gives a reactive power fluctuation in more phase delay to the phase of the reactive power in the disconnected electric power system 6 with the fluctuation being in the positive (+) polarity and the fuel cell power generation system 1 gives a reactive power fluctuation in more phase advance to the phase of the reactive power in the disconnected electric power system 6 with the fluctuation being in the negative (−) polarity, an increase in frequency as shown in (B) of FIG. 4 occurs between time t0 and t1. This operates the frequency increase monitoring circuit 22 at time t1 to make the waveform of the reactive power fluctuation at the power receiving point A as the waveform of the output of the negative polarity peak hold circuit 27 by the switching circuit 29 (see (A) of FIG. 4).
The frequency fluctuation at this time is enlarged as the fluctuation value between the time t1 and the time t2 shown in (B) of FIG. 4. Hence, at the time t2 shown in (B) of FIG. 4, the frequency upper limit monitoring circuit 30 is operated, by which a single operation phenomenon is detected by the single operation signal generating circuit 32 (see (C) in FIG. 4).    [Patent Document 1] Japanese Patent No. 3353549, corresponding to published Japanese application 08-331765:
As was explained above, in the distributed power supply system disclosed in Japanese Patent No. 3353549 (the disclosure of which is incorporated herein by reference), when the frequency increase monitoring circuit 22 is operated, a reactive current is outputted that fluctuates with the frequency fluctuation. However, reactive currents at some levels cannot promptly vary the level to the level of abnormal frequency. Thus, a reactive current with a capacity of tens of percent or more of the capacity of a converter must be outputted. At this time, however, the value of the total of an active current and the reactive current sometimes exceeded the rated value of the system to result in an overcurrent state. A measure for avoiding the overcurrent state is to enlarge the volume of the system. This, however, decreases the utilization factor of the system under normal conditions to cause a problem of making the system disadvantageous in cost and the volume of the system.